U.S. patent number 5,268,572 [Application Number 07/948,394] was granted by the patent office on 1993-12-07 for differentially pumped ion trap mass spectrometer.
This patent grant is currently assigned to Cornell Research Foundation, Inc.. Invention is credited to John D. Henion, Alex Mordehai.
United States Patent |
5,268,572 |
Mordehai , et al. |
December 7, 1993 |
Differentially pumped ion trap mass spectrometer
Abstract
An ion trap mass spectrometer includes an ion trap region
separated from an electron multiplier region by a baffle, and
separate turbomolecular pumps for pumping each region to a
different pressure level. The ion trap region can therefore be
pumped to a higher pressure level than ca be used for the electron
multiplier region, and the result and increase in pressure of
damping gas in the ion trap region increases the sensitivity of the
device.
Inventors: |
Mordehai; Alex (Ithaca, NY),
Henion; John D. (Trumansburg, NY) |
Assignee: |
Cornell Research Foundation,
Inc. (Ithaca, NY)
|
Family
ID: |
25487786 |
Appl.
No.: |
07/948,394 |
Filed: |
September 23, 1992 |
Current U.S.
Class: |
250/289; 250/290;
250/291; 250/292 |
Current CPC
Class: |
H01J
49/426 (20130101); H01J 49/24 (20130101) |
Current International
Class: |
H01J
49/02 (20060101); H01J 49/42 (20060101); H01J
49/34 (20060101); H01J 49/24 (20060101); H01J
049/24 (); H01J 049/42 () |
Field of
Search: |
;250/289,290,291,292 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Electrospray Ionization Combined with Ion Trap Mass Spectrometry"
Analytical Chemistry, vol. 62, No. 13, Jul. 1, 1990 pp. 1284-1295
Berkel et al. .
"Ion Spray Liquid Chromatography/Ion Trap Mass Spectrometry
Determination of Biomolecules" McLuckey et al., Analytical
Chemistry, vol. 63, No. 4, Feb. 15, 1991, pp. 375-383..
|
Primary Examiner: Berman; Jack I.
Attorney, Agent or Firm: Jones, Tullar & Cooper
Claims
What is claimed is:
1. A differentially pumped ion trap mass spectrometer
comprising:
a housing including a first enclosed region and a second enclosed
region;
a baffle separating said first and second enclosed regions;
an ion trap disposed in said first region;
an electron multiplier detector disposed in said second region;
first pump means in communication with said first region for
pressurizing said first region to a first pressure level;
second pump means in communication with said second region for
pressurizing said second region to a second pressure level
different from that of said first pressure level; and,
means to inject ions into said ion trap for subsequent detection by
said electron multiplier detector.
2. The spectrometer of claim 1, wherein said first pump means pumps
said first region to a pressure level of approximately
6.times.10.sup.-3 Torr, and said second pump means pumps said
second pump region to a pressure level of approximately
2.times.10.sup.-5 Torr.
3. The spectrometer of claim 1, further including a pinhole
disposed between said first and second regions for admitting ions
from said ion trap to said electron multiplier detector.
4. The spectrometer of claim 1, further including valve means for
introduction of damping gas into said first region.
5. The spectrometer of claim 4, wherein said valve means is
adjustable for varying the damping gas pressure in said first
region.
6. The spectrometer of claim 4, further including first pressure
measuring means in communication with said first region for
measuring pressure of damping gas therein and second pressure
measuring means in communication with said second region for
measuring the pressure in said second region.
7. A differentially pumped ion trap mass spectrometer
comprising:
a housing including a first enclosed region and a second enclosed
region;
means separating said first and second enclosed regions;
an ion trap disposed in said first region;
an electron multiplier detector disposed in said second region for
detecting ions from said ion trap; and,
means for pressurizing said first region to a first pressure level
and said second region to a second pressure level different from
that of said first pressure level.
8. The spectrometer of claim 7, wherein said means for pressurizing
comprises first pump means in communication with said first region
and second pump means in communication with said second region.
9. The spectrometer of claim 8, wherein said first pump means pumps
said first region to a pressure level of approximately
6.times.10.sup.-3 Torr, and said second pump means pumps said
second pump region to a pressure level of approximately
2.times.10.sup.-5 Torr.
10. The spectrometer of claim 7, wherein said means separating said
first and second enclosed regions comprises a baffle.
11. The spectrometer of claim 7, further including a pinhole
disposed between said first and second regions for admitting ions
from said ion trap to said electron multiplier detector.
12. The spectrometer of claim 7, further including valve means for
introduction of damping gas into said first region.
13. The spectrometer of claim 12, wherein said value means is
adjustable for varying the damping gas pressure in said first
region.
14. The spectrometer of claim 12, further including first pressure
measuring means in communication with said first region for
measuring pressure of damping gas therein and second pressure
measuring means in communication with said second region for
measuring the pressure ins aid second region.
Description
BACKGROUND OF THE INVENTION
The present invention relates in general to an ion trap mass
spectrometer having two separate vacuum regions allowing for use
with various ionization techniques for ion production outside the
spectrometer's ion trap mass analyzer. In particular, this
invention relates to atmospheric pressure ionization (API)
techniques (electrospray, ion spray, corona discharge etc.) for use
with the ion trap mass spectrometer for direct composition analysis
of liquids.
An ion spray interface for an ion trap mass spectrometer is the
subject of copending application, USSN 07/889,693, filed May 29,
1992, which is hereby incorporated by reference. The interface
disclosed therein permits the injection and mass analysis of ions
formed at atmospheric pressure into a quadrupole ion trap mass
spectrometer. With the interface, ions formed from a spray of
charged solvent droplets can be injected into the ion trap mass
analyzer, thereby permitting the use of condensed-phase separation
science technologies, such as liquid chromatography/mass
spectrometry (LC/MS), capillary electrophoresis/mass spectrometry
(CE/MS) and ion chromatography/mass spectrometry (IC/MS).
One difficulty encountered when using the device set forth in USSN
889,693 for API/MS applications is that of effective ion injection
and confinement in the ion trap mass analyzer.
SUMMARY OF THE INVENTION
The present invention provides an ion trap mass spectrometer
structure which enables more ions to be injected and confined
within the ion trap, thereby increasing sensitivity. To do this,
the number of damping gas (e.g. He) molecules present in the ion
trap is increased by increasing pressure in the ion trap region. As
a result, the externally produced ions collide with the helium
molecules and lose energy, thereby enabling them to be more easily
captured in the ion trap. Unfortunately, the necessary increase in
ion trap pressure to achieve this result is not compatible with the
spectrometer's electron multiplier that is commonly used to detect
the ions. To overcome this problem, the present invention employs a
baffle to separate the ion trap region from the electron multiplier
region, and separate turbomolecular pumps are employed to provide
differential pumping of the two regions.
In the preferred embodiment of the invention, the interface
disclosed in USSN 889,693 is coupled to an entrance end cap of the
ion trap system so that externally produced ions at atmospheric
pressure may be introduced into the ion trap. The differential
pumping of the mass-analyzed ion trap region and the detection
region permits a much higher helium pressure in the ion trap
region, thereby improving sensitivity of the instrument.
BRIEF DESCRIPTION OF THE DRAWING
The features and advantages of the present invention will become
apparent to those of skill in the art from the following detailed
description of a preferred embodiment thereof, taken in conjunction
with the accompanying drawing, in which:
FIG. 1 is a diagrammatic illustration of an ion trap mass
spectrometer that forms the preferred embodiment of the present
invention;
FIG. 2 illustrates the relationship between total ion current (TIC)
and pressure of helium damping gas in the ion trap region of the
mass spectrometer; and,
FIGS. 3A and 3B illustrate the mass spectrum of TBAH with helium
damping gas pressure of 6.times.10.sup.-3 Torr in the ion trap
region.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to a more detailed consideration of a preferred
embodiment of the present invention, FIG. 1 illustrates an ion trap
mass spectrometer structure 10 including a housing 12 containing a
hyperbolic ion trap analyzer 14 and an electron multiplier detector
16.
The hyperbolic ion trap analyzer 14 is contained within an ion trap
region 18 of the housing 12, while the electron multiplier detector
16 is contained within an electron multiplier region 20 of the
housing 12. The two regions 18 and 20 are separated from one
another by means of a baffle 22.
A first turbomolecular pump 24 is placed in communication with the
ion trap region 18 for pumping this region to a first desired
pressure level, while a second turbomolecular pump 26 is placed in
communication with the electron multiplier region 20 for pumping
this region to a second desired pressure level. The only
communication between the ion trap 14 and the electron multiplier
16 is by way of a pinhole 28 approximately 1 mm in diameter
disposed in the ion trap 14 which permits ions to enter the
electron multiplier region 20 for detection.
An atmospheric pressure interface 30 constructed in accordance with
the device set forth in USSN 889,693 is coupled to the ion trap
analyzer 14 for introducing externally produced ions at atmospheric
pressure into the ion trap analyzer 14. First and second ion gauge
manometers 34 and 36 are attached to the housing 12 in
communication with the ion trap region 18 and electron multiplier
region 20, respectively, for measuring the respective pressures
therein. An adjustable variable leak valve 38 is mounted on the
housing 12 for introducing damping gas, such as helium, into the
ion trap region 18.
Preferably, the first turbomolecular pump 24 is capable of pumping
500 liters per second so that the ion trap region 18 can be pumped
to a base pressure of approximately 5.times.10.sup.-5 Torr. When
damping gas such as helium, is introduced into the ion trap region
18 through the variable leak valve 38, the total pressure in the
ion trap region 18 is preferably between 5.times.10.sup.-3 and
8.times.10.sup.-3 Torr. The second turbomoleoular pump 26 is
selected to pump 60 liters per second so that the electron
multiplier region 20 can be pumped to a lower pressure of at least
approximately 2.times.10.sup.-5 Torr. The pressure ratio between
the ion trap region 18 and the electron multiplier region 20 is
therefore greater than 100:1.
To test the sensitivity and mass resolution of an ion trap mass
spectrometer constructed in accordance with the present invention,
simple infusion experiments were conducted. Tetrabutylammonium
hydroxide (TBAH), a tetraalkyl quaternary amine that exists in
aqueous solution as a cation was chosen for these studies.
Experimental conditions leading to confinement of externally
produced ions in the ion trap were also studied FIG. 2 shows the
dependence of total ion current (TIC) vs. pressure of the helium
damping gas in the ion trap. The ability of the hyperbolic ion trap
to capture externally produced ions was increased linearly with
pressure up to the pressure of 5.times.10-3 Torr, and was saturated
at a defined value in the pressure range 5.times.10.sup.-3 to
1.times.10.sup.-2 Torr. From this relationship, it is apparent that
the maximum trapping efficiency and sensitivity correspond to a
helium pressure between 5.times.10.sup.-3 and 1.times.10.sup.-2
Torr. It should be noted that normal operating conditions for an
ion trap mass spectrometer without a differential pumping system
require the pressure of helium to be below 1.times.10.sup.- 3 Torr.
Thus, the present invention is at least five times more sensitive
than would be a device having a single pressure region. The
differential pumping of the ion trap region 18 and the electron
multiplier region 20 therefore allows much higher helium pressure
in the ion trap region 18, thus improving sensitivity
substantially.
A typical mass spectrum from 1 pmol/.mu.L TBAH in methanol,
obtained under mild declustering conditions is shown in FIGS. 3A
and 3B. In particular, FIG. 3A shows that the base peak in the mass
spectrum corresponds to the molecular ion of TBAH (m/z=242). Unit
mass resolution of the .sup.12 C and .sup.13 C isotopes of TBAH
(molecular ion) was observed and is illustrated in the enlargement
of FIG. 3B. FIGS. 3A and 3B show that the present invention
provides a much cleaner (through reduced noise) mass spectrum than
can be obtained with previous mass spectrometers.
Although the invention has been disclosed in terms of a preferred
embodiment, it will be understood that numerous variations and
modifications could be made thereto without departing from the
scope of the invention as defined in the following claims.
* * * * *